Electric smelting process for production of silicon-aluminum alloys



2,755,178 Patented July 17, 1956 2,755,178 ELECTRIC SMELTING PROCESS FORPRODUC- TION F SILICON-ALUMINUM ALLOYS Robert T. C. Rasmussen, Albany,Oreg.

No Drawing. Application July 30, 1952, Serial No. 301,810

3. Claims. (CI. 7 5-10) 7 (Granted under Title 35, U. S. Code (1952),sec. 266) The invention described herein may be manufactured and used byor for the Government of the United States Y for governmental purposeswithout the payment to me of any royalty thereon in accordance with theprovisions of the act of April 30, 1928 (Ch. 460, 45 Stat. L. 467).

This invention relates to a method for the manufacture ofsilicon-aluminum alloys by reducing silica and alumina-containing rawmaterials, such as pyrophyllite,

clay, and the like, with carbon in an electric furnace.

The expression silicon-aluminum alloys as used herein'refers to alloyscontaining in excess of 50 per cent aluminum content as well as toalloys in which the silicon content exceeds the aluminum content.

Since the carbothermic reduction of aluminum and silicon oxides requiresa very high temperature, in the order of 2000 C., it has been proposedto carry out such operations by electric furnace methods, similar tothose employed for production of ferrosilicon. However, satisfactoryoperation of the electric furnace method for production ofsilicon-aluminum alloys has been extremely difiicult. Use of a chargecontaining cake in proportions theoretically required to reduce theoxides of the raw materials with formation of an equivalent amount ofcarbon monoxide proved to be unsatisfactory because of high powerrequirements, excessive electrode consumption, and unfavorableconditions within the furnace requiring frequent shut-downs. y

it has also been proposed to achieve operable conditions in asingle-phase, bottom-electrode furnace by use of a charge containingcoke in an amount providing less than the stoichiometric carbonrequirement. However, in a three-phase, top-electrode furnace, adeficiency of carbon tends to result in selective reduction of thesilica with an accumulation of an aluminum oxide slag on the furnacehearth. Continuous operation of the furnace with a charge containingcoke in either the stoichiometric quantity or in a lesser quantity isdiflicult, due to crusting of the charge accompanied by violent gasblows, re-

quiring rodding down at frequent intervals to break the tough crust andpermit descent of the charge. Losses of metal 'occur by vaporizationduring the glas blows, and the metal yield from the ore smelted iscorrespondingly low.

The use of charcoal in the charge to supply the necessary carbon forreduction eliminates crusting and gas blows, but continuous operationwith this material is unsuccessful since fused and unreduced materialcollects on the hearth of the furnace to such an extent as to requireshut-down.

Accordingly, it is an object of this invention to provide an improvedreduction process for the production of silicon-aluminum alloys in anelectric furnace.

Another object of the invention is to provide a process for the electricsmelting of silicon-aluminum alloys in which the temperature of thesmelting zone is maintained several hundred degrees Centigrade above themelting temperature of the ore.

Another object of this invention is to provide a reduction process forthe continuous production of siliconaluminum alloys in an electricfurnace in which power requirements and electrode consumption are lowand in which the amount of carbon in the charge is sufiicienttheoretically to reduce all of the oxides in the charge.

A further object of this invention is to provide a process for thecontinuous production of silicon-aluminum alloys in an electric furnacein which proper operating conditions are maintained in the furnace byproperly adjusting the charge constituents.

A still further object of the invention is to provide a process for thecontinuous production of siliconvaluminum alloys from clays and likematerials in an electric furnace with low power and electrodeconsumption in which the rate at which the ore descends through thefurnace is controlled by properly adjusting the charge constituents.

Other objects and advantages-of the invention will be apparent as thedescription proceeds.

This invention resides in an improvement in the continuouselectric-furnace carbothermic reduction process for the production ofsilicon-aluminum alloys from clay,

' pyrophyllite, or similar ores and raw materials, which comprisessupplying the furnace with ore and with carbon in at least that amounttheoretically required to reduce all of the oxides of the ore, andmaintaining a deep and porous cover of unfus ed charge over the moltenbath of the furnace and retarding the rate of descent of the ore throughthe furnace by supplying at lease a portion of the total carbonrequirement to the furnace as a bulky form of carbon, whereby thenecessary high temperature for reduction is attained and maintained inthe melting zone.

In accordance with the invention, it has been discovered that successfulcontinuous operation of the electric furnace process for the productionof siliconaluminum alloys is dependent upon the maintenance of a chargewithin the furnace covering the entire surface of the molten bath andcontaining at least the stoichiometric quantity of carbon and of suchconsistency that it will remain open for free passage of gasses and thatits ore component will descend slowly enough to permit attainment of areduction temperature in the smelting zone several hundred degrees abovethe melting temperature of the ore. The use of the bulky carbon in thecharge in proper proportions in relation to the total carbon permits themaintenance of a deep but porous cover charge and limits the rate atwhich the ore component of the charge mixture descends through thefurnace and smelting zone thereby permitting attainment of the necessaryhigh reduction temperature. Since at least the stoichiometric proportionof total carbon is present, all of the oxides are reduced and there isno undesirable formation of a slag of unreduced material to accumulateon the hearth.

Where an open and freely descending charge is maintained in the electricfurnace, as by the use of charcoal to supply the carbon, asaforementioned, lack of crusting or other means for retarding the rateof flow permits the ore to descend through the furnace as fast as it ismelted in the vicinity of the electrode tips and the necessary reductiontemperature cannot be attained. Thusfused and unreduced materialcollects on the furnace hearth.

0n the other hand, where coke is employed to supply the carbon to thecharge, crusting and bridging in the furnace-occurs to such an extentthat frequent rodding down is neceessary, the temperature mayperiodically be too low or unnecessarily high, and furnace operation isirregular.

These ditliculties are now overcome by supplying a substantialproportion of the necessary carbon to the charge in the form of woodwaste or other bulky form of carbon. Sawdust, wood chips, wood wasteknown to the timber industry as hog fuel, or a combination of thesematerials are the preferred bulky forms of carbon, but the invention isnot restricted to these materials. Other bulky carbonaceous materialsuch as corn cobs, nut shells, fruit pits, peat, and lignite also may beused as the bulky form of carbon in the smelter charge. Bulky cellulosicmaterials are generally satisfactory. The remainder of the necessarycarbon is supplied as a dense form of carbon, preferably as petroleumcoke. From the following tabulation, it is apparent that 5.7 cubic feetof wood waste is required to supply the same quantity of carbon as onecubic foot of wood charcoal, and that 18.5 cubic feet of wood waste isrequired to supply the same quantity of carbon as one cubic foot ofpetroleum coke.

Volume gr loo ftf PM m Fixed carbo percent Fuel good waste (hog fuel)Coal ar Metallurgical coke Petroleum coke Use of the correct proportionsof wood waste and coke in the smelter charge performs the vital functionof controlling the feed rate of the ore while permitting the correctproportion of total carbon for reduction to be contained in the charge.The wood waste, by virtue of its great bulk, limits the rate of descentof the ore in the furnace while at the same time permitting the chargemixture to be fed to the furnace at the necessary rate to maintain drytop conditions, thereby permitting attainment and maintenance in thesmelting zone of a reduction temperature several hundred degreescentrigrade higher than the melting point of the ore.

The wood waste also serves the function of keeping the charge open andfree from crusting and bridging, so that the charge descends freely inthe furnace and the reaction gases escape uniformly over the entiresurface of the charge. With these conditions, virtually no rodding downof the charge is required, as the charge descends gradually and inperiodic slumps, principally in the delta between the electrodes. Thethick dry top cover of charge is maintained at all times, therebyproviding excellent insulation to confine the heat to the smelting zoneand preventing loss of silicon and aluminum by vaporization. The absenceof metal fumes emanating from the furnance top, together with theabsence of slag formation, makes for extremely high metal yield and lowconsumption of power and of electrodes per pound of metal tapped fromthe furnace.

The silicon and aluminum-containing raw materials treated by thisprocess may be obtained from any suitable source. Ores higher in silicacontent than used in the Bayer process may be satisfactorily employed.Silica alone may be used with corresponding production of silicon. lfalloys high in aluminum are desired, highalumina raw material such asBayer-grade bauxite may be added to the lower grade alumina materialsor, possibly, may be used alone. Fluxing oxides may be used with thecharge as desired. It is preferred to use cheap materials, such as clayor pyrophyllite, from deposits which are low in iron and titanium. Clayswhich have been pretreated to remove iron and titanium may also be used.

The proportion of carbon in bulky form to carbon in dense form in thecharge mixture and the proportion of total carbon in the charge willvary somewhat with the nature of the silicon and alumina'containing rawmaterial. The remainder of the stoichiometric amount of carbon and anyexcess carbon is supplied in a bulky form, such as wood waste. Bestresults are obtained when the total carbon is somewhat in excess of theamount theoretically necessary to reduce all of the oxide of the ore.Too much carbon is indicated by abnormal rising of the electrode tipsand results in build-up of carbon or carbides beneath the electrodes,but adjustment of this condi tion is easier than adjustment when toolittle carbon is used. Increase in the proportion of bulky form ofcarbon above the optimum results in a higher temperature in the smeltingzone than necessary thereby resulting in higher than optimum powerconsumption.

The process is further illustrated by the following examples ofpractice. In carrying out these examples an open-top, three-phase,top-electrode furnace was employed. It is understood, however, that theinvention is not limited to this type of furnace but may be adapted toany arc-resistance type electric furnace known to the art. Although thehighest degree of process development was attained in the smelting ofpyrophyllite to produce alloy of about 23 per cent aluminum content,alloy containing as much as 60 per cent aluminum has been produced fromclay.

EXAMPLE I A charge mixture was prepared by mixing I00 pounds ofpyrophyllite with 48 pounds of charcoal. This charge contained about thestoichiometric quantity of carbon for complete reduction of silica andof all metals in the pyrophyllite. This mixture was charged into thefurnace and smelting operations were started. After about 25 hoursoperation with charges of this composition, it became certain thataccretion formation on the hearth would stop further operationsemploying this charge.

EXAMPLE 2 For a period of several days a charge containing calcined clayand petroleum coke in proportions of pounds of clay to 37 pounds ofcoke, representing about the stoichiometric carbon requirement, was fedto the furnace while carbon in excess of the stoichiometric amount wasfed to the furnance in the form of hog-fuel. The quantity of hog-fuel inthe charge was varied from time to time with a view to obtaininga chargethat was open enough to permit escape of the gases and that woulddescend freely in the furnace without undue crusting, as well as topermit enough bulk of charge to maintain a good dry-top cover in thefurnace. During a substantial portion of this period the chargecontained 100 pounds of hog-fuel in addition to the 37 pounds ofpetroleum coke and 100 pounds of calcined clay. Eventually theelectrodes failed to properly descend through the charge, indicatingaccumulation of carbon or carbide on the hearth and resulting in aninoperable condition requiring correction.

EXAMPLE 3 Pyrophyllite, petroleum coke, and hog-fuel were charged to thefurnace in proportions of50 pounds of pyrophyllite. 8 pounds ofpetroleum coke and 100 pounds of hog fuel. The coke provided about 42per cent of the stoichiometric quantity of carbon and the total carbonin the charge represented about l0l per cent of the stoichiometricamount. The furnace was successfully operated for a sustained period of96.5 hours by feeding additional charges of this composition more orless continuously as required to maintain charge level and periodicallytapping the accumulated metal from the hearth.

Materials charged and consumed Pyrophyllite, lb I4,000 Silica, lb 25Petroleum coke, lb 2,246 Hog fuel, lb 28,450 Electric power, kw.-hr41,530 Graphite electrode (approximate). lb 74l Carbon in charge, percent of stoichiometric..- 101.6 Power consumption, kw. hr. per lb. ofore smelted 2.96 Power consumption, kw. hr. per lb. of alloy produced6.35 El ctmde consumption, lb./ton of alloy produced. 22:

Silicon-aluminum alloy produced Uncontaminated alloy as tapped, lb 4,639Alloy contaminated with iron (90% of actual weight), lb. 1,901

Total metal, adjusted weight, lb 6,540 Total alloy yield, per cent 100.4Silicon recovered in alloy, per cent 2 95.8 Aluminum recovered in alloy,per cent 2 101.7 Average analysis of alloy:

Al, per cent 22.6 Si, per cent 71.4 Fe, per cent 1.8 Ti, per cent 0.76

The iron in the contaminated metal melted from the iron bar used to openthe tap hole or to clear hole during tapping. Ninety per cent of theweight of the iron-contaminated metal was arbitrarily used in summing upthe total metal produced.

2 Exact average aluminum and silicon analyses are difiicult to establishbecause of analytical ditficulties and because part of the alloyproduced was contaminated with iron during tap ping. However, thefigures given are reasonably accurate, and recoveries of both aluminumand silicon are better than 95 per cent.

During the operation of the process with the charge proportions as inExample 3, the furnace operated very smoothly, the electrodes descendedproperly through the charge, there was'no evidence of vaporization loss,and no accretion formation on the hearth.

It is evident from the foregoing description and examples that properadjustment of the charge proportions, in accordance with this invention,with use of wood waste or similar bulky carbonaceous material to providea part of the required quantity of carbon provides conditions forsmooth, trouble-free smelting operations in\the furnace in which thecharge mixture needs only to be fed to the furnace at the proper rate tomaintain the desired charge level and in which the product alloy istapped from the furnace periodically.

The process of this invention is useful in providing silicon andaluminum for casting alloys. In this use the product will serve as amaster alloy to provide the silicon and part of the aluminum containedin the commercial aluminum-silicon alloys. The product can also be usedas a deoxidizer and reducing agent in the refining and extraction ofother metals. Aruminum-silicon alloys are also becoming increasinglyimportant as new methods for separating the aluminum and silicon fromthe alloys are developed.

It will be appreciated from a reading of the foregoing specificationthat the invention herein described is suscepbelow its reductiontemperature, the improvement which comprises sufiiciently diluting theraw material in said mixture with bulky carbonaceous matter to retardthe rate at which the raw material enters the smelting zone so that thetemperature of the smelting zone is increased to the required reductiontemperature for the raw material, said dilution being a minimum of about5.6 cubic feet of said carbonaceous matter per 100 pounds of said rawmaterial and such volume of said carbonaceous matter containing not morecarbon than the correct amount required for the reduction of said rawmaterial.

2. The process according to claim 1, wherein the alumina andsilica-containing raw material is calcined clay and the bulkycarbonaceous matter is hog fuel, and

in which the mixture contains about 100 parts by Weight of said hog fuelto parts by Weight of said calcined clay plus enough petroleum coke tofurnish the remainder of the required carbon reductant, said mixturecontaining about 5.6 cubic feet of said hog fuel per 100 pounds of saidcalcined clay.

3. The process according to claim 1, wherein the alumina andsilica-containing raw material is pyrophyllite and the bulkycarbonaceous matter is hog fuel, and in which the mixture contains about200 parts by weight of said hog fuel to 100 parts by Weight of saidpyrophyllite plus enough petroleum coke to furnish the remainder of therequired carbon reductant, said mixture containing about 11.2 cubic feetof said hog fuel per 100 pounds of said pyrophyllite.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN THE PROCESS OF CONTINUALLY PRODUCING SILICONALUMINUM ALLOY IN ANELECTRIC FURNACE BY REDUCTION OF ALUMINA AND SILICA-CONTAINING RAWMATERIAL WITH CARBON, IN WHICH A MIXTURE OF SAID RAW MATERIAL AND CARBONIS FED ONTO AND CONTINUOUSLY MAINTAINED UPON A SMELTING ZONE, HEAT ISCONTINUOUSLY APPLIED TO THE MIXTURE IN THE SMELTING ZONE, AND MOLTENPRODUCT IS TAPPED FROM THE FURNACE, SAID RAW MATERIAL HAVING A MELTINGTEMPERATURE BELOW ITS REDUCTION TEMPERATURE, THE IMPROVEMENT WHICHCOMPRISES SUFFICIENTLY DILUTING THE RAW MATERIAL IN SAID MIXTURE WITHBULKY CARBONACEOUS MATTER TO RETARD THE RATE AT WHICH THE RAW MATERIALENTERS THE SMELTING ZONE SO THAT THE TEMPERATURE OF THE SMELTING ZONE ISINCREASED TO THE REQUIRED REDUCTION TEMPERATURE FOR THE RAW MATERIAL,SAID DILUTION BEING A MINIMUM OF ABOUT 5.6 CUBIC FEET OF SAIDCARBONACEOUS MATTER PER 100 POUNDS OF SAID RAW MATERIAL AND SUCH VOLUMEOF SAID CARBONACEOUS MATTER CONTAINING NOT MORE CARBON THAN THE CORRECTAMOUNT REQUIRED FOR THE REDUCTION OF SAID RAW MATERIAL.